This invention relates to ethylene polymer compositions especially useful in making cable insulation for high voltage applications. The composition contains an additive which provides resistance to electrical breakdown.
Electrical breakdown of high voltage insulation, known as dielectric failure, is often initiated at the sites of cavities and contaminating particles. Despite extreme care used in making, handling, and extruding ethylene polymer insulation, cavities and contaminants can be introduced in any step prior to final shaping. The breakdown of insulation in high voltage applications is known to the trade as electrical "treeing". Electrical treeing is a rather slow progressive degradation of an insulation composition caused by electron and ion bombardment of the insulation and resulting in the formation of microchannels or tubes having an overall tree-like appearance. Trees are initiated at locations of cavities or contaminants by the action of ionization (corona) during high voltage surges. Once initiated, trees usually grow, hastened by voltage surges, until such time as dielectric failure occurs.
Another phenomenon which may cause electrical breakdown is water treeing. Water trees are different in appearance from electrical trees. They have a diffuse and indistinct appearance; they do not appear to be branched or to be channels. They are believed by some researchers to be microcracks or minute water agglomerates. They are found only in cables that have been exposed to normal operating electric stresses in a moist or wet environment. Water trees, like electric trees, are initiated at cavities and contaminating particles. It has been suggested that as water trees grow, they can become electrical trees as part of the ultimate electrical breakdown.
To overcome the problem of treeing, various additives have been proposed, particularly for use in polyethylene or polyolefins, which additives serve to either prevent formation of trees or delay tree growth. Certain alcohols have been found to be very effective additives for delay of tree growth in ethylene polymer insulation. See U.S. Pat. No. 4,206,260 to E. J. McMahon. However, the alcohol content decreases with exposure of the insulation to elevated temperatures. Exudation of alcohol can be mitigated but not prevented by addition of polypropylene, as described in U.S. Pat. No. 4,283,459 to Urban et al.
Other additives which have been proposed for ethylene polymer insulation to increase resistance to electrical breakdown include: an inorganic salt of a strong acid with a strong zwitterion compound in U.S. Pat. No. 3,499,791 to Maloney; a ferrocene compound with a substituted quinoline compound in U.S. Pat. No. 3,956,420 to Kato et al.; silicone fluid in U.S. Pat. No. 3,795,646 to McKenzie, Jr.; and an aromatic ketone in Japanese Patent No. 14348/75 to Fujikura Cable Works, Ltd.